In astronomy, the Pleiades (pron.: /ˈplaɪ.ədiːz / or /ˈpliː.ədiːz /), or Seven Sisters ( Messier object 45 or M45), is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky. The name Pleiades comes from Greek mythology; the celestial entity has several meanings in different cultures and traditions.

The cluster is dominated by hot blue and extremely luminous stars that have formed within the last 100 million years. Dust that forms a faint reflection nebulosity around the brightest stars was thought at first to be left over from the formation of the cluster (hence the alternate name Maia Nebula after the star Maia), but is now known to be an unrelated dust cloud in the interstellar medium that the stars are currently passing through. Computer simulations have shown that the Pleiades was probably formed from a compact configuration that resembled the Orion Nebula. Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighbourhood.

Observational history

The Pleiades are a prominent sight in winter in the Northern Hemisphere and in summer in the Southern Hemisphere, and have been known since antiquity to cultures all around the world, including the Māori, Aboriginal Australians, the Persians, the Chinese, the Japanese, the Maya, the Aztec, and the Sioux and Cherokee. In Tamil culture this star cluster is attributed to Lord Murugan (Lord Murugan raised by the six sisters known as the Kārththikai Pengal and thus came to be known as Kārtikeyan). In Sanskrit he is known as Skanda.

The Nebra sky disk, dated c. 1600 BC. The cluster of dots near the upper right portion of the disk is believed to be the Pleiades.

The Babylonian star catalogues name the Pleides MUL.MUL or "star of stars", and they head the list of stars along the ecliptic, reflecting the fact that they were close to the point of vernal equinox around the 23rd century BC. The earliest known depiction of the Pleiades is likely a bronze age artifact known as the Nebra sky disk, dated to approximately 1600 BC. Some Greek astronomers considered them to be a distinct constellation, and they are mentioned by Hesiod, and in Homer's Iliad and Odyssey. They are also mentioned three times in the Bible ( Job 9:9 and 38:31, as well as Amos 5:8). The Pleiades ( Krittika) are particularly revered in Hindu mythology as the six mothers of the war god Murugan, who developed six faces, one for each of them. Some scholars of Islam suggested that the Pleiades (Ats-tsuraiya) are the Star in Najm, which is mentioned in the Quran.

In Japan, the constellation is mentioned under the name Mutsuraboshi ("six stars") in the 8th century Kojiki and Manyosyu documents. The constellation is also known in Japan as Subaru (“unite”) and is depicted in the logo of the Subaru automobile company. The Persian equivalent is Nahid (pronounced "Naheed").

The rising of the Pleiades is mentioned in the Ancient Greek text Geoponica. The Greeks oriented the Hecatompedon temple of 1150 BC and the Parthenon of 438 BC to their rising.

They have long been known to be a physically related group of stars rather than any chance alignment. The Reverend John Michell calculated in 1767 that the probability of a chance alignment of so many bright stars was only 1 in 500,000, and so correctly surmised that the Pleiades and many other clusters of stars must be physically related. When studies were first made of the stars' proper motions, it was found that they are all moving in the same direction across the sky, at the same rate, further demonstrating that they were related.

Charles Messier measured the position of the cluster and included it as M45 in his catalogue of comet-like objects, published in 1771. Along with the Orion Nebula and the Praesepe cluster, Messier's inclusion of the Pleiades has been noted as curious, as most of Messier's objects were much fainter and more easily confused with comets—something that seems scarcely possible for the Pleiades. One possibility is that Messier simply wanted to have a larger catalogue than his scientific rival Lacaille, whose 1755 catalogue contained 42 objects, and so he added some bright, well-known objects to boost his list.

Edme-Sébastien Jeaurat then drew in 1782 a map of 64 stars of the Pleiades from his observations in 1779, which he published in 1786.

Distance

Comet Machholz appears to pass near the Pleiades in early 2005

An amateur photograph taken with a DSLR camera.

The distance to the Pleiades can be used as an important first step to calibrate the cosmic distance ladder. As the cluster is so close to the Earth, its distance is relatively easy to measure and has been estimated by many methods. Accurate knowledge of the distance allows astronomers to plot a Hertzsprung-Russell diagram for the cluster, which, when compared to those plotted for clusters whose distance is not known, allows their distances to be estimated. Other methods can then extend the distance scale from open clusters to galaxies and clusters of galaxies, and a cosmic distance ladder can be constructed. Ultimately astronomers' understanding of the age and future evolution of the universe is influenced by their knowledge of the distance to the Pleiades. Yet some authors argue that the controversy over the distance to the Pleiades discussed below is a red herring, since the cosmic distance ladder can (presently) rely on a suite of other nearby clusters where consensus exists regarding the distances as established by Hipparcos and independent means (e.g., the Hyades, Coma Berenices cluster, etc.).

Measurements of the distance have elicited much controversy. Results prior to the launch of the Hipparcos satellite generally found that the Pleiades were about 135 parsecs away from Earth. Data from Hipparcos yielded a surprising result, namely a distance of only 118 parsecs by measuring the parallax of stars in the cluster—a technique that should yield the most direct and accurate results. Later work consistently argued that the Hipparcos distance measurement for the Pleiades was erroneous. In particular, distances derived to the cluster via the Hubble Space Telescope and infrared color-magnitude diagram fitting favour a distance between 135–140 pc. However, the author of the 2007–2009 catalog of revised Hipparcos parallaxes reasserted that the distance to the Pleiades is ~120 pc, and challenged the dissenting evidence. Recently, Francis and Anderson proposed that a systematic effect on Hipparcos parallax errors for stars in clusters biases calculation using the weighted mean, and gave a Hipparcos parallax distance of 126 pc, and photometric distance 132 pc based on stars in the AB Doradus, Tucana-Horologium moving group and Beta Pictoris moving groups, which are similar in age and composition to the Pleiades. Those authors note that the difference between these results can be attributed to random error.

Composition

X-ray images of the Pleiades reveal the stars with the hottest atmospheres. Green squares indicate the seven optically brightest stars.

The cluster core radius is about 8 light years and tidal radius is about 43 light years. The cluster contains over 1,000 statistically confirmed members, although this figure excludes unresolved binary stars. It is dominated by young, hot blue stars, up to 14 of which can be seen with the naked eye depending on local observing conditions. The arrangement of the brightest stars is somewhat similar to Ursa Major and Ursa Minor. The total mass contained in the cluster is estimated to be about 800 solar masses.

The cluster contains many brown dwarfs, which are objects with less than about 8% of the Sun's mass, not heavy enough for nuclear fusion reactions to start in their cores and become proper stars. They may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of the total mass. Astronomers have made great efforts to find and analyse brown dwarfs in the Pleiades and other young clusters, because they are still relatively bright and observable, while brown dwarfs in older clusters have faded and are much more difficult to study.

Age and future evolution

Ages for star clusters can be estimated by comparing the Hertzsprung-Russell diagram for the cluster with theoretical models of stellar evolution. Using this technique, ages for the Pleiades of between 75 and 150 million years have been estimated. The wide spread in estimated ages is a result of uncertainties in stellar evolution models, which include factors such as convective overshoot, in which a convective zone within a star penetrates an otherwise non-convective zone, resulting in higher apparent ages.

Another way of estimating the age of the cluster is by looking at the lowest-mass objects. In normal main sequence stars, lithium is rapidly destroyed in nuclear fusion reactions. Brown dwarfs can retain their lithium, however. Due to lithium's very low ignition temperature of 2.5 million kelvin, the highest-mass brown dwarfs will burn it eventually, and so determining the highest mass of brown dwarfs still containing lithium in the cluster can give an idea of its age. Applying this technique to the Pleiades gives an age of about 115 million years.

The cluster is slowly moving in the direction of the feet of what is currently the constellation of Orion. Like most open clusters, the Pleiades will not stay gravitationally bound forever. Some component stars will be ejected after close encounters with other stars; others will be stripped by tidal gravitational fields. Calculations suggest that the cluster will take about 250 million years to disperse, with gravitational interactions with giant molecular clouds and the spiral arms of our galaxy also hastening its demise.

Reflection nebulosity

Under ideal observing conditions, some hint of nebulosity may be seen around the cluster, and this shows up in long-exposure photographs. It is a reflection nebula, caused by dust reflecting the blue light of the hot, young stars.

It was formerly thought that the dust was left over from the formation of the cluster, but at the age of about 100 million years generally accepted for the cluster, almost all the dust originally present would have been dispersed by radiation pressure. Instead, it seems that the cluster is simply passing through a particularly dusty region of the interstellar medium.

Studies show that the dust responsible for the nebulosity is not uniformly distributed, but is concentrated mainly in two layers along the line of sight to the cluster. These layers may have been formed by deceleration due to radiation pressure as the dust has moved towards the stars.

Brightest stars

The nine brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Sterope, Merope, Electra, Maia, Taygeta, Celaeno, and Alcyone, along with their parents Atlas and Pleione. As daughters of Atlas, the Hyades were sisters of the Pleiades. The English name of the cluster itself is of Greek origin, though of uncertain etymology. Suggested derivations include: from πλεîν pleîn, to sail, making the Pleiades the "sailing ones"; from pleos, full or many; or from peleiades, flock of doves. The following table gives details of the brightest stars in the cluster:

A map of the Pleiades

The location of the Pleiades on the constellation Taurus.

Pleiades Bright Stars

Name

Pronunciation ( IPA & respelling)

Designation

Apparent magnitude

Stellar classification

Alcyone

/ælˈsaɪ.əniː/al-SY-ə-nee

Eta (25) Tauri

2.86

B7IIIe

Atlas

/ˈætləs/AT-ləs

27 Tauri

3.62

B8III

Electra

/ɨˈlɛktrə/i-LEK-trə

17 Tauri

3.70

B6IIIe

Maia

/ˈmeɪə/, /ˈmaɪə/MAY-ə, MY-ə

20 Tauri

3.86

B7III

Merope

/ˈmɛrəpiː/MERR-ə-pee

23 Tauri

4.17

B6IVev

Taygeta

/teɪˈɪdʒɨtə/tay-IJ-i-tə

19 Tauri

4.29

B6V

Pleione

/ˈplaɪ.əniː/PLY-ə-nee

28 (BU) Tauri

5.09 (var.)

B8IVpe

Celaeno

/sɨˈliːnoʊ/sə-LEE-noh

16 Tauri

5.44

B7IV

Sterope, Asterope

/ˈstɛrɵpiː/, /əˈstɛrɵpiː/(ə)-STERR-ə-pee

21 and 22 Tauri

5.64;6.41

B8Ve/B9V

—

—

18 Tauri

5.65

B8V

Possible planets

Analyzing deep-infrared images obtained by the Spitzer Space Telescope and Gemini North telescope, astronomers discovered that one of cluster's star - HD 23514, which has a mass and luminosity a bit greater than those of the sun, is surrounded by an extraordinary number of hot dust particles. This could be an evidence for planets formation around HD 23514.